Common service names are not allowed to be used as a User Definable Custom Login Name. Following are examples of common service names:

abrt
account
add
adm
admin
administrator
anon
apache
api
app
apps
audio
auth
avahi
backup
bin
cdrom
centos
cgred
chrony
console
consul
core
coreos
crontab
daemon
data
dbus
dev
dialout
dip
disk
docker
dockermon
error
errors
fax
finger
floppy
ftp
fuse
games
gnats
google
gopher
haldaemon
halt
haproxy
help
home
host
html
http
httpd
imap
info
irc
kerberos
kmem
landscape
libstoragemgmt
libuuid
list
lock
lp
mail
maildrop
mailer-daemon
mailerdaemon
mailq
man
master
me
mem
messagebus
mlocate
modem
moderator
mx
mysql
mysqld
netdev
new
news
nfs
nobody
nntp
nobody
nogroup
ns
ns1
ns2
ns3
ns4
ntp
null
oauth
oauth_clients
openid
opensuse
operator
oprofile
owner
passwd
password
pdns
pdns-recursor
php
plugdev
polkitd
polkituser
postdrop
postfix
postgres
postgresd
postmaster
proxy
pub
public
qpidd
redhat
rightlink
rightscale
root
rpc
rpcuser
sasl
saslauth
sftp
shadow
shutdown
sles
slocate
smtp
snmp
src
ssh
sshd
ssl
staff
stapdev
stapusr
sudo
super
superuser
support
sync
sys
sysadmin
syslog
systemd-bus-proxy
systemd
network
tape
tcpdump
test
trusted
tss
tty
ubuntu
undefined
user
username
users
utempter
utmp
uucp
uuidd
vcsa
video
voice
web
webmail
webmaster
website
websites
wheel
whoopsie
www
www-data
wwwrun
xfn
xmpp
xok
you